The present invention generally relates to an improved spark plug for an internal combustion engine, and more particularly, to a spark plug having an ablative coating deposited on the end portion of its ceramic insulator which ablates to remove contaminants which are deposited thereon prior to sustained use or firing of the spark plug.
During an initial period of time prior to when an internal combustion vehicle, such as a car or truck, is sold to a user, the vehicle engine is repeatedly subjected to conditioning running and/or short-time running during which low speed and low loads are experienced. For example, transporting the vehicle from the factory to a dealer involves loading and unloading the vehicle onto and from a truck requiring operation of the vehicle for only short distances and time intervals. Typically, spark plugs used in internal combustion engines include a ceramic insulator having an end portion exposed to the interior chamber of the engine. This end portion or tip of the ceramic insulator can become contaminated with a wide variety of substances, such as carbon or soot, which interfere with the operation of the spark plug in that the spark plug has difficulty in producing a strong spark. The spark plug is extremely susceptible to such contamination during the initial life of the vehicle. Such effects are even more prevalent when the vehicle is subjected to cold climates. In order to guard against this type of contaminative damage, heat-resistant insulating materials have been deposited on the surface of the end portion or tip of the ceramic insulator which is exposed to the interior of the combustion engine with a view toward preventing leakage of current from the surface of the ceramic insulator end portion to the metal housing of the spark plug.
Past attempts in the art provide coatings which are durable, permanent and designed to survive the life of the spark plug. For example, Ishino, U.S. Pat. No. 4,937,484, discloses a heat-resistant coating for application to the end portion of the ceramic insulator. The coating is formed of a solution of silicone oil, paraffin and ozokerite combined in a manner which increases the coating's viscosity. Such a coating is clearly designed to be permanent and durable, so as to prevent contaminants such as carbon or soot from fouling or otherwise interfering with the sparking capabilities of the spark plug.
The problem experienced with spark plugs having such permanent and durable coatings, however, is that the coatings have a tendency to increase the temperature of the end portion or tip of the spark plug beyond the range for which the spark plug was designed. This is especially prevalent during later usage of the vehicle and at high vehicular speeds. Thus, it would be desirable to have a spark plug which resists contaminant fouling during the initial life of the vehicle, yet does not interfere or inhibit performance of the spark plug during later or sustained use.
Accordingly, there is a need in the art for a spark plug which resists contaminant fouling during the initial life of the spark plug, thereby improving the operation of the vehicle. There is also a need in the art for such a spark plug which does not interfere with or inhibit performance during later or sustained operation of the vehicle.